1. Field of the Invention
This invention relates to frequency trimming of surface acoustic wave resonators, and more particularly, to SAW resonators having a precisely set resonant frequency and a method for laser trimming of metallizations to produce the desired frequency characteristic in such devices.
2. Description of the Prior Art
Many applications of surface acoustic wave (SAW) devices require designs which yield a very precisely set frequency. One type of such devices is the SAW resonator which offers the potential for high frequency operation of narrowband filtering and oscillators for a variety of electronic systems, such as those used in both military and commercial systems for radio communication and radar signal processing. Although these resonators can be used in many systems that require frequency control and/or signal processing devices, of particular interest is the use of SAW resonators for filtering for a variety of mobile radio communication systems. In these devices, it is desirable to have enough flexibility in the design to provide a low cost, simple means of producing a small precise shift in frequency.
Several prior art techniques have been employed for fine tuning of SAW resonators. In a paper entitled "Fine Tuning of Surface Acoustic-Wave Resonator Filters with Metallization Thickness" by Haydl et al, Electronics Letters, June 12, 1975, Vol. 11, No. 12, pages 252-253, a technique of controlling the frequency of the resonator by controlling metallization thickness is described. This technique is somewhat cumbersome, since the entire crystal must be immersed in an acid etch to remove metallization, or the crystal must be placed in a vacuum chamber to add or remove metallization. Another prior art technique is that of using argon ion bombardment for fine tuning of SAW resonators, as reported in a paper titled "Fine Tuning of SAW Resonators Using Ion Bombardment," by James et al, in Electronics Letters, Oct. 11, 1979, Vol. 15, No. 21, pages 683-684. This technique requires the resonator sample to be placed in a chamber, where either a change in mass loading of the quartz surface or a change in impedance due to a difference in sputtering rates of aluminum and quartz is carried out. A prior art technique to trim SAW resonator center frequencies is described in a paper by Rosenfeld et al, entitled "Tuning Quartz SAW Resonators by Opening Shorted Reflectors," in the 1977 Proceedings of the Thirty-First Annual Frequency Control Symposium, pages 231-239. In this technique a laser is used to open-circuit grating fingers by etching to shift the device frequency upwards. Yet another prior art technique for frequency trimming uses a deposition or overlay of dielectric films to alter the electric field associated with the propagating surface waves. Control of deposition of dielectric films is difficult, because placing the entire device inside a vacuum chamber and deposition by sputtering is required. These requirements, along with controlling environmental factors, such as heat, complicates control of the process, making very precise frequency adjustment extremely difficult. None of these prior art techniques offers a very precise frequency shifting technique, and each of them requires extra equipment and processing steps adding complexity and cost to the manufacture of SAW devices.